1. Field of the Invention
The present invention relates to a lithium secondary battery, more particularly improvement of the capacity and cycle-characteristic of a lithium secondary battery.
2. Related Background Art
The so-called lithium secondary batteries comprising a positive electrode with lithium cobaltate as a major active material, negative electrode with carbon as a major active material and an organic electrolyte solution have been put on the markets since the beginning of the 1990's. They have been rapidly spreading in the markets since then, because of their higher capacity than that of a conventional nickel/hydrogen secondary battery and sufficient cycle-characteristic to satisfy the market needs. At the same time, extensive works have been done to improve their characteristics and develop batteries of higher capacities.
As a result, the cylindrical battery of 18 mm in diameter and 65 mm in height, the so-called 18650 size, has now a capacity of 2,200 mAh at the highest, comparing with around 1,000 mAh recorded in the beginning of the 1990's. The greatly enhanced capacity results from improvements in a wide area including materials, e.g., lithium cobaltate and carbon as active materials, and designs.
However, it is considered that current capacity of a lithium ion secondary battery with lithium cobaltate and carbon as major active materials is close to the limit. Therefore, new active materials have been studied for the positive and negative electrode as another approach to higher capacity.
In particular, for the negative electrode active materials, metallic materials that can be alloyed with lithium, e.g., silicon and tin, have been studied as substitutes for carbon materials such as graphite. This is because they have greater theoretical capacities which are 3 to 10 times that of graphite such that while the theoretical capacity capable of charging/discharging of graphite is 372 mAh/g, a silicon alloy (Li4.4Si) has a theoretical capacity of 4,199 mAh/g and a tin alloy (Li4.4Sn) has a theoretical capacity of 993 mAh/g.
However, some metallic materials that can be alloyed with lithium involve their own problems to be solved, because they may expand during the alloying reaction process to increase the negative electrode volume several times, which tends to powder them, resulting in deterioration of their cycle-characteristic.
Several proposals have been made to solve these problems, as disclosed by U.S. Pat. Nos. 6,051,340, 5,795,679 and 6,432,585, Japanese Patent Application Laid-Open Nos. 11-283627 and 2000-311681 and WO 00/17949.
For example, U.S. Pat. No. 6,051,340 proposes a lithium secondary battery with a negative electrode comprising a current collector coated with an electrode layer, wherein the current collector is of a metal which is not alloyed with lithium, and the electrode layer comprises a metal which can be alloyed with lithium, such as silicon or tin and another metal which is not alloyed with lithium, such as nickel or copper.
U.S. Pat. No. 5,795,679 proposes a lithium secondary battery with a negative electrode formed of an alloy powder comprising an element such as nickel or copper and another element such as tin; and U.S. Pat. No. 6,432,585 a battery with a negative electrode whose electrode material layer contains at least 35% by weight of silicon or tin particles having an average particle diameter of 0.5 to 60 μm, a void ratio of 0.10 to 0.86 and a density of 1.00 to 6.56 g/cm3.
Japanese Patent Application Laid-Open No. H11-283627 proposes a lithium secondary battery with a negative electrode containing silicon or tin having an amorphous phase; and Japanese Patent Application Laid-Open No. 2000-311681 a lithium secondary battery with a negative electrode composed of amorphous tin/transition metal alloy particles of a non-stoichiometric composition. WO 00/17949 discloses a lithium secondary battery with a negative electrode composed of amorphous silicon/transition metal alloy particles of a non-stoichiometric composition.
Moreover, Japanese Patent Application Laid-Open No. 2000-215887 proposes a lithium secondary battery whose capacity and charge/discharge efficiency are improved by suppressing the volume expansion during alloying with lithium to prevent the breakage of the negative electrode, wherein chemical vapor deposition involving pyrolysis of benzene or the like is used to solve the above problems by forming a carbon layer on the surface of particles of a metal or semi-metal, in particular silicon, which can be alloyed with lithium, to improve its electroconductivity.
These inventions have disclosed compositions and constituents of silicon or its alloys, and performance of the electrode that comprises the above material. It should be noted, however, that a battery exhibits its inherent functions when its negative electrode works in combination with a positive electrode, both contained in a battery can. For a battery to exhibit its intended functions, it is essential to design a battery of high capacity and cycle-characteristic by allowing a negative electrode mainly composed of a metallic material which can be alloyed with lithium to effectively function in a battery can in combination with a positive electrode.
Japanese Patent Application Laid-Open No. 2002-352797 proposes a lithium secondary battery of high capacity and cycle-characteristic by controlling utilization of a negative electrode comprised of silicon at a certain level or less. However, it only discloses silicon coated with carbon for a negative electrode, discussing that recommended electrical storage capacity (hereinafter, simply referred to as “storage capacity” or “capacity”) per unit weight of a negative electrode active material layer is 1,000 mAh/g, but is silent on conditions for extending the life of a battery having a capacity exceeding 1,000 mAh/g.
In other words, few have sufficiently discussed optimum electrode and battery designs that allow a battery to exhibit a high capacity and a long cycle life when it comprises a negative electrode of a high capacity per unit weight of a negative electrode active material layer exceeding 1,000 mAh/g working in combination with a positive electrode.